In the handling and fabrication of polymers to be cross-linked, substantial problems may be encountered in view of the "scorchy" nature of the compositions. The tendency of such compositions to prematurely cross-link or vulcanize together with a short shelf life will result in the necessity for an exacting schedule for the fabrication operation. Even with such a rigorously maintained schedule for the fabrication of materials, loss of material through premature vulcanization is often encountered.
A conventional technique for the compounding of polymers to be subsequently cross-linked is a two-pass mixing cycle utilizing a mixer such as a Banbury mixer. In the first cycle of this process, the bulk of the ingredients to be present in the final cross-linked product, i.e. polymer, fillers, processing aids, antioxidants, etc., with the exception of the cross-linking agent itself are incorporated into a homogeneous base compound in a high-shear mixer such as a Banbury mixer. The heat generated during this first cycle is so great that the addition of the cross-linking agent would initiate the cross-linking and would result in a "scorched" composition. That is, the mechanical energy transferred to the material being mixed increases the heat content of the mass, resulting in composition temperatures as high as 170.degree. C. and the addition of cross-linking agents to these base compositions while in their high energy state would result in their immediate vulcanization. Thus, in the second stage of this technique, the cross-linking agent is added to the product of the first cycle at a lower temperature, e.g. about 90.degree. C., in a two-roll mill or in a mixer such as a Banbury mixer. This accelerated composition, i.e., having the cross-linking agent in admixture therewith, is then cooled, dried and possibly stored to await final fabrication such as by extrusion and heating to effect cross-linking.
However, this two-pass mixing cycle technique has significant disadvantages. Storage, even at room temperature, may result in some degree of cross-linking or thermosetting of the polymer. The time requirements which are imposed upon this process in view of the premature cross-linking are significant and may require strict compounding schedules which would require an undesirable inflexibility in the use of available equipment. Even with such a schedule, prematurely cross-linked batches which cannot be used for the desired final product may be generated.
Although a method has been proposed in U.S. Pat. No. 3,923,947 for the introduction of a cross-linking agent into polyethylene in order to overcome some of these disadvantages, a need exists for a reliable process to cure these problems in general.
At present, many vulcanizable compositions based on neoprene (polychloroprene), hypalon (chlorosulfonated polyethylene), nitrile rubber (acrylonitrile butadiene copolymer), SBR (styrene butadiene copolymer), and butyl rubber, exhibit such short shelf-life that inter-plant shipments of these compositions from the point of preparation to the point of final conversion is limited to short distances. This leads either to duplication of manufacturing effort and equipment requirements or to the specialization of manufacturing plants which at times causes restrictive inflexibility of overall manufacturing capabilities.
Therefore, an object of the present invention is a process for the preparation of cross-linkable polymeric compositions with low levels of "heat history", i.e., polymeric compositions with the cross-linking agent in admixture therewith yet low levels of cross-linking.
A further object of the present invention is a process for the preparation of polymeric compositions which have long shelf lives yet need only be extruded before the cross-linking operation itself.
A further object of the present invention is a process for the preparation of cross-linkable polymeric compositions by which the generation of unusable scrap is reduced.
An additional object of the present invention is the preparation of polymeric compositions containing a cross-linking agent whereby inter-plant transportation of such compositions would be possible as the result of an increased storage shelf life. This factor would allow the decentralization of various equipment or the utilization of existing facilities without the requirement that machinery such as mixers and extruders be present in one location.
A still further object of the present invention is a process for the preparation of compositions which are more tolerant to variances that occur within final fabrication equipment such as extruders.